1. Field of the Invention
The present invention relates to a method and an apparatus for detecting a start point of electroless plating such as electroless Co-P plating and electroless Co-Ni-P plating, and more particularly to a method and an apparatus for detecting the start of electroless plating, which can be suitably used for controlling the film thickness in electroless plating.
2. Description of the Prior Art
Electroless plating such as electroless Co-P plating and electroless Co-Ni-P plating has been employed widely in the production of magnetic memories or the like as means for producing magnetic films. In such electroless plating, the control of the film thickness is a matter of utmost importance since the thickness of a plated film has great influence on the physical properties of the product. Especially, in the case of a magnetic film which is formed by electroless Co-P plating or electroless Co-Ni-P plating, it is important to detect the start of plating for the control of the film thickness as the film is normally plated in a very small thickness of about 0.1 .mu.m and within a very short time period of about 40-60 seconds. In this connection, it has been the general practice to control the film thickness by way of the plating time. When forming a film of a relatively large thickness over a long plating time, the difference in film thickness is less than 0.1 .mu.m between the case where the plating is initiated immediately after immersion in the plating solution of an object or material to be coated and the case where the plating is initiated several seconds or ten and several seconds or several tens of seconds after immersion of the object to be coated depending upon the activeness of the plating solution. Namely, when plating thick films, no objectionable errors occur in film thickness even if the plating time is regard as the time of immersion of an object to be coated in a plating solution, that is to say, the length of time between the time point when the object is immersed in the plating solution and the time point when the object is lifted from the plating bath. Accordingly, the film thickness can be controlled on the basis of the immersion time of the object to be coated instead of the plating time, without knowing the exact starting point of plating after immersion of the object. However, in the case of the above-mentioned electroless plating in which normally a film with a thickness of about 0.1 .mu.m is plated within a time period of about 40-60 seconds, the substitution of the plating time for the immersion time is perilous since the plating may start with a delay of serveral to several tens of seconds after immersion of an object to be coated in a plating solution. Thus, in view of the difficulty of controlling the film thickness by way of the immersion time, it is necessary to detect the exact starting point of plating and to control the plating time accordingly in order to control the film thickness appropriately.
As for the method for detecting the start of plating, although it is conceivable to use a calomel electrode or silver-silver chloride electrode as a reference (comparison) electrode thereby measuring the difference of potential from the object, this method entails troublesome problems in the maintenance and service of the electrode and the use of complicated apparatus.